How to copy paste parts of Themes Setups from one theme to another?
Well as far as I know there is no builtin mechanism to do this, at least not directly through the blender UI or in a graphical manner.

You can copy individual colors from one theme to another, by hovering your mouse cursor over a color box and pressing Ctrl + C and Ctrl + V, but I don't think this is what you expect.

Other than than, Blender themes are simple XML files, you can edit them externally using any regular plain text editor.

You can theoretically copy and paste at will between them. If you use some sort of programmer's text editor with syntax highlighting for XML files and possibly code folding for easier navigation it should make the job somewhat easier.